The present invention concerns an ion selective part of an apparatus for the determination of the concentration of ions in liquid media. The ion selective part contains a polymeric material and an ion selective component having a selectivity for the ions to be determinated. The ion selective part may optionally contain a plasticizer.
Corresponding apparatuses for the determination of ion concentrations are used in many fields of the art. The polymeric material of the corresponding ion selective parts is usually polyvinylchloride. Furthermore many ion selective components having a selectivity for specific ions are described in the art. Also, several plasticizers for corresponding ion selective parts are described in publications. When the prior art apparatuses were used for the determination of the concentration of ions in aqueous media which were free of organic constituents having a comparatively high molecular weight, then usually accurate results are achieved and after a calibration of the equipment the corresponding results were also reliable and had a good reproducibility. If, however, the ion concentrations had to be determined in liquid media which contained organic constituents of comparatively high molecular weight and specially corresponding organic constituents which also comprise hydrophilic groups, such as proteins or proteinous materials, then the reproducibility of the determination of the ion concentrations was often insufficient. These deficiencies are due to the fact that the standard potential E.sub.o of the Nicolsky-Eisenmann equation shifts after the corresponding ion selective parts are in contact with the liquid medium containing proteinous materials. Furthermore, through such a contact an asymmetry potential is developed.
Since several years extended research work was done in order to find plasticizer components through which the above-stated deficiency of corresponding ion selective membranes comprising a polymeric material, an ion selective component and a plasticizer would be avoided.
It is the object of the present invention to solve the problem in a quite different way, i.e. by modifying the polymeric component of the corresponding ion selective parts so that the above stated deficiencies are avoided. It, accordingly, is the object of the present invention to provide ion selective parts which can be used in corresponding apparatuses for the determination of ion concentrations in biological material, such as body fluids, as for instance blood serum and whole blood. The corresponding ion selective parts should have a high stability of the standard potential and a low asymmetry potential or no asymmetry potential at all after they had been in contact with the corresponding liquid media containing biological constituents.